CAN bus message is removed when many nodes are present

I would guess that the CAN bit rate on your nodes is slightly different than that of the third party nodes. By default (usually) non-passive nodes on a CAN bus monitor every packet on the bus. When the node sees a valid packet (even if it will not be received by the node) it will ACK the packet. If the node sees a malformed packet, it will actively transmit to ensure the packet is not received by any other node.

I would start by putting an oscilloscope on the CAN lines and measure the bit rate that your nodes are transmitting at. Then, if possible, measure the bit rate at which the third party nodes transmit at. You may well find that either your nodes are not transmitting at the rate you expected, or that the third party nodes are transmitting a rate you do not expect.

Mat is on the right track, but you need two 120 ohm terminating resistors, one at each physical end of the bus. The bus should be a single set of wires not a pair as your drawing indicates. Measure the bus resistance when it is off, it should be between 55 to 65 ohms. It needs to be a bus not a star configuration. I am not use to emitters on the bus. The emitters must allow dominate and recessive modes. All messages must be acknowledged, if they are not they will eventually error and time out. They must all be operating at the same baud. You can get an inexpensive USB logic analyzer and monitor the bus to see what is happening. If you have access to a scope that would even be better. Either of these will tell you if everything is at the same baud. I have used a lot of MPC2515 modules and over time and vendors they have had different crystal values. Here is a link to some of the basics of CAN that might help: "https://grouper.ieee.org/groups/msc/upamd/communications/CAN_Primer.pdf" Mixing protocols is a receipe for a problem. I would suggest you start at a lower baud rate, maybe 125K or less and get everything to work with one type of message, 11 or 29 bit identifier. I do not understand why there are receivers only, the software can keep them from transmitting.

From Keil Slide linked above: " Any node see something wrong on the bus. Makes bus dominant. All nodes knows this is an error frame. Sender stops transmitting. Increments its Transmit Error Counter by 8. If TEC < FF, resends message else busoff. Others increment their Receive Error Counters. Note: only a node can boot itself off the bus." I thought this might help it sounds close to what is happening. Different bauds will cause errors.